1. Field of the Invention
The present invention concerns a current mirror electronic circuit, whose architecture was designed with a view to achieving good performance at a low voltage close to the lower voltage supply, and a low conducting resistance. By modifying a known mirror, to which is added a feedback circuit, the output current is held constant regardless of the voltage applied across the terminals of the mirror according to the invention.
The invention can be applied to circuits built with different types of transistors: in order to make the description clearer, the invention will be explained with reference to a circuit consisting of N-MOS transistors, but the scope of the invention is not limited to this.
2. Discussion of the Background
The current mirror is well known in itself in analog electronics, and the basic drawing is shown in FIG. 1. In a very simplified form, between two sources of voltages V.sub.DD and V.sub.SS are positioned:
a reference branch comprising a current source 1, supplying a current I, and a first transistor T1,
a tracking or tracing branch comprising a load 2 and a second transistor T2. The gates of T1 and T2 are connected to each other and to the current source 1, so that the current I' which crosses the load 2 tracks the current I of source 1.
In fact, this type of current mirror has an error (I'=I) due to the transistor gain, especially at low gain. This can be corrected for by creating a Wilson mirror, schematized in FIG. 2, in which a transistor T3 is added on the tracking branch, and its gate is connected to the reference branch, between the source 1 and T1. The transistor T3 receives feedback by means of a simple mirror. In this type of assembly, the voltage excursion of point A, between the load 2 and T.sub.3, is limited to a few 100 mV+V.sub.GS above the "low" voltage V.sub.SS. "A few 100 mV" corresponds to the voltage drop across T.sub.3, and V.sub.GS to the voltage drop across T2.
In the improved Wilson mirror in FIG. 3, a transistor T.sub.4 added in the reference branch allows T.sub.1 to work under the same conditions as T.sub.2, making the circuit symmetrical, because the pair T.sub.3, T.sub.4 applies the same voltage at points B and C, improving the tracking of the current. However, both types of Wilson mirror have two transistors in series in the tracking branch.
Thus, the two types of Wilson mirror described only work for output voltages (at A) greater than V.sub.SS +V.sub.GS + a few 100 mV, a value which is too high in certain cases, taking into account that, for MOS transistors, V.sub.GS can reach values as high as 4 or 5 volts, while the circuits operate at 1 volt.
This limitation is illustrated by the curves in FIG. 4 which show the current/voltage characteristics of a mirror according to different gate/source voltages V.sub.GS, for the output transistor T.sub.2. The dotted curves such as 5 correspond to a simple current mirror (FIG. 1) and the solid line curves correspond to a Wilson current mirror (FIGS. 2 and 3). It can be seen that Wilson mirrors only reach a saturation (and therefore stable) current I.sub.DSsat for a value of V.sub.DS, which, at point (7), is higher than for a simple mirror, (point 8). The arrows 9 show the differences which exist, for a given voltage V.sub.GS, between a simple mirror and a Wilson mirror: for the latter type, the tracking is better but at the cost of a higher V.sub.DSS.
V.sub.DS or V.sub.DSS is called the threshold value, which in former embodiments is much greater than V.sub.SS because in the tracking branch there are two transistors T2 and T3 in series, whose conducting resistance R.sub.on is too high.